The present invention relates generally to a photosensor and more particularly concerns a photosensor device for use in facsimile equipment, optical character recognition systems and the like.
A known type facsimile transmitter reads out information from an original or document by means of a photosensor or image sensor disposed in close contact with the document. The use of the contact type linear image sensor makes it unnecessary to optically reduce the size of document, so that an optical system otherwise required can be dispensed with.
Typical examples of such image sensors are disclosed in U.S. Pat. No. 4,227,078, U.S. Pat. No. 4,233,506 and others. In FIG. 1 of the accompanying drawings, there is shown in a perspective view a contact type linear image sensor, in which a reference numeral 1 denotes an original or document, 4 denotes an illuminating light source, and 8 denotes generally a photosensor device. An arrow 2 represents the direction in which the document 1 is scanned.
A hitherto known structure of the contact type linear image sensor in which optical fibers 9 are used for picking up image information is shown in FIGS. 2a and 2b of the accompanying drawings in a schematic sectional view and a top plan view, respectively. In these figures, a reference numeral 1 denotes a document, 4 denotes an illuminating light source, 8 denotes a base plate or substrate, 10 denotes photosensors, 11 denotes IC's (integrated circuits) for driving the photosensor elements 10, and a reference numeral 13 denotes a rotatable roller. On the other hand, FIGS. 2c and 2d show in a schematic sectional view and a top plan view, respectively, a hitherto known structure of the contact type linear image sensor in which photodiodes 10 are used in place of the optical fibers as the information reading sensor elements and are adapted to be positioned in direct contact with the document. In these figures, a reference numeral 15 denotes a substrate, 11 denotes IC's for driving photosensor elements 10, and a numeral 13 denotes a rotatable roller. In the case of the image sensor structure shown in FIGS. 2c and 2d, light transmissive windows 14 are formed in the substrate 15, being interposed between the photodiodes 10 in alignment therewith, for illuminating the document 1 with light emitted from a light source 4. FIG. 3 shows an exemplary arrangement of a scanning circuit used in combination with the linear image sensor device illustrated in FIGS. 2a to 2d. Referring to FIG. 3, each of scanning IC's 11 includes MOS transistors 16 serving as gates for selecting the photodiodes and a shift register 17 for sequentially driving these gates 16. The photodiode is represented by an equivalent circuit in this figure as indicated by a circle 10 and assumed to be composed of a photo-current source 18 for producing a current varying in dependence on the quantity of incident light and an equivalent capacitor 19. A reference numeral 20 denotes a source of bias voltage applied to the photodiodes 10.
Operations of photo-electrical conversion and the data read-out are carried out in the manner described below. Immediately after the pick-up of image data from a document, circuit points 22 are set to the ground potential by way of the selecting transistors 16, as the result of which the capacitors 19 are charged to the bias voltage V.sub.T by the bias voltage source 20. Subsequently, the transistors are turned off, whereby the circuit points 22 are disconnected from the ground potential. In this state, the capacitors 19 are discharged through the photo-current sources 18. In this manner, carriers as generated are stored in the capacitors 19. The signals representing quantities of carriers stored in the individual capacitors 19 can be read out through a common line 12 sequentially as the MOS transistors 16 are sequentially turned on under the control of the shift registers 17, 17' and 17". The scanning system of this type requires a number of transistors 16 which corresponds to the number of picture elements represented by the photodiodes as well as a corresponding number of the driving stages 17 for driving the photodiodes. For example, in the case of the contact type image sensor having 1760 picture elements (photodiodes), as many as twenty-two IC's are required on the assumption that the single scanning IC is assigned with 80 picture elements to be scanned. This makes the image sensor quite expensive. Further, the number of pin contacts required for each of the scanner IC's amounts in total to about 90 inclusive of 80 pins for connection to the picture elements or photodiodes and about 10 pins for connection to the power supply and the like. Under these circumstances, when twenty-two IC's are to be incorporated in the single image sensor of the contact type as mentioned above, 1980 (=90.times.22) connections will be required. Such high density of the connections required for the IC packages demands extremely complicated and sophisticated manufacturing procedures, involving great difficulties in implementation of the image sensor for practical applications. This problem can be readily appreciated when one considers the fact that the number of connections required in a conventional IC package amounts to at most about twenty.